The outer membrane (OM) of a Gram negative bacterium is its interface with the outside world. Proteins in this membrane act as selective barriers to block harmful substances (such as drugs or viruses) and pass critical nutrients. OM protein expression is environmentally regulated: shifts in temperature, acidity, and exposure to antibiotics cause dramatic changes in OM protein composition. Protein expression is traditionally only studied in bulk, measuring an average over large numbers of bacteria. Little is known about OM heterogeneity within the population. In fact, non-genetic variation is present and causes behavioral differences in the response of bacteria to antibiotic treatment. Non-genetic variation acts as a hedge, sacrificing the optimization of some cells under current conditions for the ability of these cells to regenerate the population in the event that conditions change. The causes of behavioral variation are unknown, but the established link between OM composition and antibiotic resistance suggests a role for outer membrane heterogeneity. This research aims to (1) quantify cell-to cell variation in OM expression, (2) test several possible mechanisms by which the population may generate this variation, and (3) measure the contribution of this natural variation to bacterial survival under changing conditions including antibiotic treatment. PUBLIC HEALTH RELEVANCE: The threat from infectious disease, currently the leading cause of premature death worldwide [1], is compounded by the increasing frequency and severity of antimicrobial resistance and rise in the rate of failure of antibiotic treatments [2]. In order to reduce the growing toll on human life taken by bacterial infections, it is first necessary to understand how bacteria survive when challenged by antibiotic treatment. The goal of this research is to quantify the contribution of non-genetic diversity in bacteria to survival and adaptability in the face of antibiotic stress. 1. The Global Burden of Disease: 2004 Update. Edited by: World Health Organization;2008. 2. Talbot GH, Bradley J, Edwards JE, Jr., Gilbert D, Scheld M, Bartlett JG: Bad bugs need drugs: an update on the development pipeline from the Antimicrobial Availability Task Force of the Infectious Diseases Society of America. Clin Infect Dis 2006, 42:657-668.